This invention relates to a computer system with a bus structure, wherein a bus control unit controls the traffic on the bus, and further including a cental processor which is connected to the bus, a storage unit which is at least indirectly connected to the bus, and a number of peripheral apparatus control units and associated peripheral devices which are connected to the bus.
Presently bus-structured computer systems receive substantial attention. Particularly in the field of small computer systems there is a tendency to utilize a bus for the interconnection of the various units, the aim being to connect the units to the bus by way of a standard interface. The bus can then have a universal and simple construction, which increases the flexibility of the entire computer system. Adaptation of the computer system to users' demands for all sorts of applications is thus possible.
The principle of the invention relates to so-called bus-organized computer systems in general, and is not restricted to a given form of such a computer system. In view of the desire to develop computer systems which are suitable for a wide variety of users, it is an increasingly stringent demand that the effort required for the design and writing of application programs be minimized. To this end, it is necessary that the instructions given to a programmer for writing a program are as simple as possible. This is to be understood as follows: the processing of data in a computer system involves many complex situations where the program must provide correct control. Such a complex situation occurs notably in the case of input/output instructions, particularly the input/output instructions where peripheral devices requiring complex control are involved. Relevant examples are: disc stores storage units and data communication connections. If the programmer has only an inadequate set of instructions for the for the control of such a complex situation, an additional effort is required in building up and writing the programs. Thus far proposals have been made which in essence facilitate the task of the programmer by the development of instructions which are simple and still serve for the control of complex situations. However, in practice this means that the problem is shifted to the level of the system programs, i.e. the programs which are usually developed by the manufacturer of the computer systems and which provide the general control, notably also for input/output instructions.
The development and continuous follow-up (training for use and assistance of the users by the supplier) of system programs, however, is also a very expensive matter and often substantial effort must be made to find optimum solutions.
Contemporary developments in the field of hardware, however, offer the possibility of shifting the described problem to the hardware.
Read-only memories are now available in many designs, also for repeated writing. Particularly read-only memories designed according to integrated circuit techniques offer substantial possibilities: high speed, reliability, small volume, low energy dissipation. Using these memories, extensive application of microprogramming is possible, and many program functions can be taken over by the microprogram control in a computer system.
The present invention relates to a proposal to utilize, within bus-structured computer systems, the described possibilities offered by the new developments in the field of hardware in a very special manner. Moreover, according to the set-up of the invention a distribution of functions between the various sections of a bus-structured computer system is achieved on an efficient basis. This is important in view of costs: functions which can be centrally performed on the basis of "sharing" in the central processor must also be performed as much as possible in this processor. An optimum compromise must then be made in view of the occupation of the central processor on the one hand, and the multiple spreading, depending on the number of peripheral devices, of the hardware on the other hand.